high reving in large displacement engines

[2.2 Straight six]

i've helped friends work on a 20.9-litre Mercedes V12 diesel (1,221ci) and that revved to something like 2,500rpm and no higher. i think it made peak torque at about 1,900rpm and peak power at 2,400rpm.

[MetalHeadZaid]

cany anybody say LONG GEAR RATIOS!

[UncleBob]

Originally Posted by 534BC

Going to a bigger bore will net more hp even if cylinder filling is not enhanced.

Going to larger main spacing and larger bore centers assumes a larger valved head.

As far as stroke goes most engines are relatively square for easy figurin' , but an oversquare (larger bore than stroke ) is almost always better for power.

Another way to say it is that increasing bore size increases power and increasing stroke does not (assuming a mean piston speed constant)

this is the problem with such generic discussion. Going with larger engine (bore and/or stroke) will definitely increase power, both torque and HP, but it moves both peaks lower, due to head flow (assuming it stays the same).

If you increase bore AND valve size/ports to take advantage to the increased combustion chamber size, then you can definitely net a more linear power increase (more power than just cc increase accounts for)

If you only increase stroke, there is no valve size increase option that you didn't have before.

The most basic principle to motors is, the smaller the engine, the more power per cc potential. This is why, per cc, radio controlled engines have unbelievable power to cc ratios (even after taking into consideration they are 2 stroke alcohol/nitro engines). The smaller the engine, the better air flow dynamics potential. Going another step up in size, motorcycle engines are breaking the 200hp/liter in stock trim, NA. very oversquare engines, lots of head flow, lots of RPM's. Very small cc per cylinder. Much further down the scale is such huge engines as the one mentioned in the beginning of this thread. Huge cc/cylinder, probably undersquare (assuming), very bad head flow for its size. (realistically) low RPM's. It all would spell out less impressive per liter numbers.

But that is way too simplistic of a comparison also. When you start comparing severe oversquare to severe undersquare, 2 valve to 4 valve, multi cylinder to less than multi cylinder....lots and lots of variables start coming into play.

To put it simply. As far as peak power is concerned, its cc's, head flow potential, and RPM's. Those three variables are the nitty gritty of performance. If you have good specs on 2 of those items, and lack luster on the third, you will have a three leg stool with a missing leg compared to what you COULD have with simular, but even distribution between the variables

[drunken monkey]

to what extent does the extra weight of having a larger bore (and thus larger/heavier piston) play a part?

[UncleBob]

effects the realistic fpm the pistons can acheive before bad things happen.

[2.2 Straight six]

Originally Posted by drunken monkey

to what extent does the extra weight of having a larger bore (and thus larger/heavier piston) play a part?

it's like having a weight on the end of a rod. if you took such a thing, and used one hand as a guide and the other to move it back and forth you can do this easily up to a point. if you go too fast the sudden change in velocity (velocity being speed with direction, not speed.) puts strain on the rod. if this change is too sudden and too violent then the rod snaps. (it can snap on both the outward and inward stroke)

so it's the same as a con-rod breaking inside an engine (except without the side-to-side movement of the crank's rotation)

i hope that helps.

[2.2 Straight six]

Originally Posted by UncleBob

this is the problem with such generic discussion. Going with larger engine (bore and/or stroke) will definitely increase power, both torque and HP, but it moves both peaks lower, due to head flow (assuming it stays the same).

yes, torque will definately increase. horsepower won't always. horsepower being calculated. and it's the work done as opposed to a force that's measured. a high revving engine with less torque can make more horsepower than a bigger engine with more torque that can't rev so high.

so you may increase engine size and see a gain in torque, but if it can't rev as highly (or there's a big drop in torque higher up in the rev range) then it may not produce as much horsepower.

[UncleBob]

Originally Posted by 2.2 Straight six

yes, torque will definately increase. horsepower won't always. horsepower being calculated. and it's the work done as opposed to a force that's measured. a high revving engine with less torque can make more horsepower than a bigger engine with more torque that can't rev so high.

so you may increase engine size and see a gain in torque, but if it can't rev as highly (or there's a big drop in torque higher up in the rev range) then it may not produce as much horsepower.

that is true, but its unlikely that you'd have a situation that was that extreme. Lowering your redline slightly on most engines, won't effect the peak HP point, since peak HP isn't usually at peak RPM.

[drunken monkey]

well... i was aware of the efect of extra weight on the piston but was trying badly to allude to the use of balancing shafts in certain "regular" engines.
i know that in the context of this discussion it is largely redundent (refering to the 780ci engine) but well, what i'm now trying to ask is; is it possible to engineer from scratch a big bored engine to rev high?

also how big can you go before a carefully designed injection of fuel/mist becomes a rough squirt (if that makes any sense)?

[UncleBob]

counterbalancing only reduces vibration, it doesn't reduce the mass of the recipricating components (it actually adds to it) which is what limits redline.

injectors (assuming you use them) are limited by opening time, as far as what RPM's they can handle, which gives them a smaller window to achieve the quanity of spray they need.....but this can be overcome by fast injectors, large injectors, or even multiple injectors. Bikes rev to 16.5K RPM's these days using injectors...and then there's F1 cars over 23K RPM's

[534BC]

Originally Posted by drunken monkey

well... i was aware of the efect of extra weight on the piston but was trying badly to allude to the use of balancing shafts in certain "regular" engines.
i know that in the context of this discussion it is largely redundent (refering to the 780ci engine) but well, what i'm now trying to ask is; is it possible to engineer from scratch a big bored engine to rev high?

also how big can you go before a carefully designed injection of fuel/mist becomes a rough squirt (if that makes any sense)?

A larg bore engine rev limit will still be determined by the previous items mentioned. A grossly oversquare engine may be a high revver, but a short stroke may be hard to get some adequate compression ratio.

If you took my 903 cid v-8 as an example and cut the stroke in half theoretically you can double the rev limit. The piston speed would be identical and amid a bunch of variables theoretically would produce the same hp as the 903, but do it at twice the rpm and be 452 cid.

hope that came out right,.

[jveik]

basically, if you want a torquey engine, concentrate on lower rpm's and very long strokes.

if you want high horsepower, focus on high rpm's and large bores and short stroke

this can be seen with a 383 chevy versus a 377 chevy (smallblocks) the 383 is a 350's 4 inch bore with .030 more bored out, and a 400's 3.75 inch stroke. it is great for low rpm torque and is an easy 500lb/ft of torque.

the 377 is a 400's bore (4.125 inches i think) with a 350's stroke (3.48inch) this motor cant make as much streetable torque, but it makes a lot more actual horsepower because the shorter stroke lets it rev much higher than the 383. i have heard that a 350 bottom end (3.48inch stroke) is good for about 6500 rpm with no modifications.

[GreyGoose006]

the problem with huge bores is that it takes longer for the flame to spread and ignite. if you want to engineer an engine from scratch and use a large bore (over 4.5 inches) you would be looking at huge valves and probably twin sparkplugs.

[UncleBob]

Originally Posted by jveik

i have heard that a 350 bottom end (3.48inch stroke) is good for about 6500 rpm with no modifications.

350 is capable of way more RPM's than that...but its a question of good parts and smart engine building. But back to my earlier formula, 3.48 x 2 / 12 = .58
1/.58/5000fpm = 8620 RPM's at a fairly aggressive build.

Now....a 283 (3" stroke) would do 10K RPM's. An old fart I work with used to race a 283 he would spin up to 10K every run. Motor held together for years with nothing breaking on him....very impressive!

[jveik]

yeah them little 283's are rockets, but anyways, were you saying that a stock bottom end 350 with no mods can hit over 8000 rpm? or is that with h-beam rods, forged cross-drilled crank, and forged pistons?

also, i have heard of people making the main bearing size smaller to allow more rpm... its like being farther out on a merry-go-round, you are moving faster than a person farther in towards the middle. i think that might be the principle behind that main bearing re-sizing.